Thermoplastic polyesters such as polyethylene terephthalate and the like have excellent impact resistance, heat resistance and transparency as well as a certain degree of gas barrier property, and have been widely used for producing a variety of kinds of packaging containers.
Such packaging containers can be represented by a container with a flange obtained by molding a stretched or unstretched thermoplastic polyester sheet.
Japanese Unexamined Patent Publication (Kokai) No. 53852/1984 discloses a method of producing a transparent container by monoaxially stretching a thermoplastic resin sheet while maintaining the reduction ratio of the width of the sheet to be not larger than 10% and heat-molding the thus obtained monoaxially oriented sheet (prior art 1).
Japanese Examined Patent Publication (Kokoku) No. 27850/1989 discloses a method of heat-molding a polyester sheet by molding a biaxially stretched polyester sheet having a crystallinity of not larger than 30% and an index of surface orientation of from 0.02 to 0.15 by utilizing the compressed air along a mold heated at a temperature which is not higher than the crystallizing temperature (Tc° C.) of the polyester but is not lower than (Tc-70)° C., heat-treating the obtained molded article by bringing it into contact with the heated mold, fitting a cooling mold to the heating mold, the cooling mold having a shape nearly corresponding to the heating mold, forcibly transferring the molded article toward the cooling mold side from the heated mold side by blowing the compressed air, and cooling the molded article upon contact with the cooling mold (prior art 2).
Japanese Examined Patent Publication (Kokoku) No. 36534/1992 discloses a polyester container having a heat-adhering portion that can be thermally adhered to the closure member, the container being obtained by molding a polyester sheet containing a polyethylene terephthalate as a chief constituent component, the heat-adhering portion having a crystallinity of smaller than 20%, and the bottom portion and(or) the side portion of the container having the crystallinity of not smaller than 20%, the container being useful as an ovenable tray (prior art 3).
Japanese Patent No. 2947486 discloses a method of producing a biaxially stretched thermoplastic product by forming a biaxially stretched intermediate product by blow-molding a sheet-like thermoplastic material in a tube at a stretching temperature while preventing the material from adhering to the top of the side walls, placing the intermediate product on a male mold of a preset size and a texture, heating the intermediate product and the mold at a temperature higher than the temperature for stretching the thermoplastic material so that the intermediate product is thermally shrunk on the surface of the mold, cooling the intermediate product that is thermally shrunk, and taking the thermally shrunk intermediate product out of the mold (prior art 4).
The prior art 1 uses a monoaxially stretched sheet as the sheet for molding. This molding method may be capable of improving the transparency of the container but still leaves room for improvement concerning the heat resistance of the container.
The prior art 2 uses a biaxially stretched sheet as the sheet for molding. This molding method may be capable of improving the heat resistance of the container but is not still satisfactory concerning the impact resistance of the container.
These prior arts 1 and 2 use a sheet that has been stretched in advance as the sheet to be molded and, hence, require a particular stretching step and, hence, an additional cost. It is therefore desired to use an unstretched sheet and to impart, in a step of forming the container, the molecular orientation that is desired from the standpoint of imparting the container properties. It is further desired that the properties such as heat resistance, impact resistance and transparency are imparted in the steps of molding the container without requiring any particular step.
According to the prior art 3, an amorphous polyester sheet that is heated and plasticized is formed into a tray by using a metal mold maintained at a crystallizing temperature in order to heat-crystallize the bottom portion and/or the side portion. However, there is no disclosure concerning molecularly orienting the side portion by stretching, and it is considered that the container that is obtained is still insufficient with respect to impact resistance and transparency.
The prior art 4 is to produce a final container by preparing a biaxially stretched intermediate product by the blow-molding and by heat-shrinking the intermediate product on the male mold. This method, however, requires both heating for heat-shrinking the intermediate product on the male mold and cooling for shaping the heat-shrunk intermediate product and for taking it out. Therefore, this method is not still satisfactory from the standpoint of thermal economy, extended periods of time occupying the molds and low productivity.
According to this production method, further, the remainder (so-called skeleton portion) of the sheet after the container-forming portion is cut off occupies a considerable proportion, usually, 40 to 60% of the sheet. Namely, the remainder is wastefully discarded lowering the yield of the material. It can be considered to reuse the remainder of the sheet by melting it deteriorating, however, the quality of the material. To avoid excess drop in the quality of the material, the sheet can be used only partly but not the whole of the remainder. To solve this problem, Japanese Unexamined Patent Publication (Kokai) No. 5-69478 and Japanese Examined Patent Publication (Kokoku) No. 7-67737 propose methods of molding a cup-like thermoplastic resin container by forming a preform by the injection molding and, then, vacuum-molding or compressed air-molding the preform being assisted with a plug. According to the method described in Japanese Unexamined Patent Publication (Kokai) No. 5-69478, however, the theroplastic resin is not heat-set and, hence, the heat resistance is poor. According to the method described in Japanese Examined Patent Publication (Kokoku) No. 7-67737, on the other hand, the heat resistance is imparted but extended periods of time are needed for the heat-setting and for cooling the heat-molded container accompanied by a problem of low production efficiency.
There has further been widely used a cup-like container made of a thermoplastic resin having a flange, a cylindrical side wall hanging from the inner edge of the flange and a bottom wall closing the lower end of the side wall. Prior to being filled with the content, the cup-like containers with a flange after molded are stacked in a plural number by utilizing the flange portions. Usually, a plurality of the cup-like containers are stored or transported in a stacked state where the flange portion and the stacking portion which is a step formed on the barrel of the container are engaged with each other (e.g., see Japanese Unexamined Patent Publication (Kokai) No. 5-213358).
However, the cup-like container having the stacking portion is produced by a method by which a polyester sheet heated at a temperature higher than a glass transition temperature is crystalled and press-stretched by using a male plug and a female cavity heated at a temperature higher than the glass transition temperature, the air pressure is applied to the softened sheet, the softened sheet is moved from the male plug to the female mold and is brought into contact with the female mold so as to be heat-set upon being heated to be not lower than the glass transition temperature, the air pressure is released, and the sheet is shrunk back onto the male plug and is cooled. In this case, however, the flange portion which has been crystallized by heat or crystallized by orientation cannot be shrunk back, and the inner diameter of the mouth which is the inner peripheral diameter of the flange cannot be formed to be sufficiently small with respect to the diameter of the step (stacking portion) formed relying upon the difference in the inner diameter of the plug barrel. As a result, the overlapping distance (=(outer stacking diameter−inner diameter of the mouth)/2) becomes small between the upper container and the lower container at the stacking portion in the stacked state. Only about the thickness of the stacking portion can be maintained as the overlapping distance at the greatest, and the stacking performance becomes poor.